1. Field of the Invention
This invention relates to an efficient method for preparing heat-curable silicone rubber compounds.
2. Prior Art
Heat-cured silicone rubber is improved in weather resistance, durability, heat resistance, physiological activity and colorability. It is used in a variety of applications including building materials, electronic materials, electric materials, business machine parts, automotive parts, and medical parts.
A variety of methods are known for the preparation of heat-curable silicone rubber compounds which heat cure into silicone rubbers. Most customarily, they are prepared using large size, open kneader/mixers having a pair of mixing blades in a chamber.
The kneader/mixers have the advantage that a large amount of mass can be produced in a single pass on account of its maximum capacity reaching several thousands of liters, but the drawback that a long time is needed until the mixture becomes homogeneous, and specifically, the mixing step takes several hours to several tens of hours. The throughput rate at which heat-curable silicone rubber compounds are prepared by means of kneader/mixers can be increased by increasing the size of the apparatus or using a multi-stage apparatus. These modifications, however, are difficult to implement partially because of limits on the installation space.
With respect to the preparation of heat-curable silicone rubber compounds, a number of methods have been investigated for the purpose of improving productivity. For example, JP-A 56736/1989 discloses a continuous production process comprising the step of feeding a diorganopolysiloxane gum to a twin-screw kneader/extruder along with an inorganic filler which has been surface treated with an organic silicon compound under pressure. The drawback of this method is that the pretreatment of the filler takes a long time. JP-A 102007/1990 discloses a continuous production process comprising the steps of continuously pulverizing raw materials in a high-speed shearing machine and feeding them to a continuous twin-screw kneader/extruder. In order to obtain uniform particles, the pulverizing conditions must be strictly controlled. This makes the process complex.